A Hybrid Automatic Repeat Request (HARQ) is a physical layer (layer one) retransmission function in the WiMax IEEE 802.16 protocol (with variants in several cellular protocols, such as in UMTS). HARQ is a link adaptation technique where link layer acknowledgements are used for re-transmission decisions at the physical layer. For each burst transmitted in the link, a receiver may or may not receive the burst properly, due to varying channel conditions. If the burst is received properly, an acknowledged (ACK) message is returned. If the burst is not received properly, a not-acknowledged (NACK) message is returned. Upon receipt of an ACK, further data bursts can be sent. Upon receipt of a NACK a retry attempt can be made to resend the same burst information.
IEEE 802.16 defines two levels of retry attempts. The first as described above is HARQ on the physical layer. The second is on a higher layer protocol such as standard Automatic Repeat Requests (ARQ) on the data plane, or Media Access Control (MAC) timers on the control plane. As used herein, MAC layer refers to any higher layer protocol such as the control plane, data plane, or application layer. In an example, if a data burst is sent and neither an ACK or NACK is returned, or if NACKs are continually received with no retry attempts being successful (i.e. HARQ failure), it must be assumed after some period of waiting in the higher layer that the burst was not received properly. In this example, a timer on a higher layer protocol determines that there is a problem, but only after a considerable amount of time may have passed.
Unfortunately, there is no feedback mechanism defined between the physical layer and higher layers (such as the control or data plane). Typically, the retransmission at the physical layer is done independently of other higher layer MAC procedures such as ARQ or handling of MAC management messages. As a result, any HARQ failure is not directly linked to handling at the upper layer protocols, which depend upon their own retransmission mechanisms (timers, ARQ, TCP, etc) to attempt another transmission. In addition, some transfers do not have a retry mechanism and the burst loss must simply be tolerated. In addition, since HARQ does not rebuild bursts when retry attempts are made, the coding scheme for that burst cannot be changed to adapt to changing channel conditions.
Referring to FIG. 1, four cases are shown demonstrating problems that arise due to HARQ failures. In Case 1, a control plane sends a MAC management message to the physical layer which experiences HARQ failure. In this example, the RF channel conditions are changing rapidly, and the inability of HARQ to modify the coding scheme on retransmissions during the changing channel conditions makes it unlikely that HARQ will be able to successfully deliver that burst within its maximum number of retry attempts. The control plane does not know of any problems until a control plane timer timeouts, which may be much later than the time HARQ abandoned the delivery attempt. Only at this point does the control plane determine that the MAC management message must be resent, which can result in a significant delay.
In Case 2, packet data units (PDU) are to be sent from a higher layer using ARQ through the physical layer. In this case, message feedback is sent whenever a peer decides to send it. There is no time limit for the peer to respond with either an ACK or NACK, which means any transmission failure resulting in a NACK can significantly delay the resending of the data packets, even if HARQ detected a delivery failure after maximum retries.
In Case 3, a higher layer is to provide ARQ feedback. If the physical layer is unable to send the feedback (i.e. HARQ failure), the higher layer must rely on future feedback attempts to continue data transfers, which can result in a significant delay.
In Case 4, packet data units (PDU) are to be sent from a higher layer through the physical layer without using ARQ. If the physical layer is unable to send the feedback (i.e. HARQ failure), data retransmission must occur at the application layer at endpoints (e.g. TCP), which can result in a significant delay.
HARQ retransmissions also results in other deleterious effects, even when a HARQ burst is eventually acknowledged. In particular, HARQ retransmission cause delays that can upset synchronize communications between higher layer protocols of a mobile station and a base station, for example.
Referring to FIG. 2, two scenarios are shown wherein HARQ retransmissions cause delays that can upset synchronization involving bases stations and mobile stations. In Case 1, during handover there is an action time negotiated between the base station and the mobile station that defines when the mobile station will arrive at a target base station. In particular, the source base station will send a mobility base station handoff request (MOB_BSHO-REQ) to the mobile station and wait for the mobile station to send a handoff indication message indicating that the mobile station would like to move to the target base station. The target base station will use a non-contention based Fast Ranging procedure which allocates an information element in the uplink MAP. The action time sent in a mobility base station handoff response (MOB_BSHO-RSP) message should be interpreted correctly between the base station and mobile station. However, due to poor RF conditions the allocation made for the handover indication message may have been lost and the mobile station may not be able to use the non-contention based Fast Ranging IE at the target base station and might need to fall back to performing contention based bandwidth request to send the handover indication successfully. During this time handover latencies can be significantly increased.
Therefore, there is a need for a technique to provide HARQ feedback (such as for HARQ transmission failures) to higher layer protocols, so that much faster and beneficial recovery mechanisms and synchronization can be immediately instituted to prevent lost data or unnecessary retransmissions.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.